In the combustion of a fuel, such as coal, oil, natural gas, peat, waste, etc., in a combustion plant, such as a power plant or a waste incineration plant, a process gas is generated. For separating nitrogen oxides, usually denoted NOx, from such a process gas, often referred to as a flue gas, a method is frequently used in which a reducing agent, usually ammonia or urea, is mixed with the flue gas. The flue gas, mixed with said ammonia or urea, is then passed through a catalyst to promote a selective reaction of the reducing agent with the NOx to form nitrogen gas and water vapour. Usually the catalyst is installed in what is commonly called a selective catalytic reduction (SCR) reactor. The mixing of the reducing agent and the flue gas is accomplished in a gas duct in a position upstream of the SCR reactor.
The reducing agent is supplied to the gas duct by a plurality of nozzles arranged within the gas duct. To facilitate an even distribution of the concentration of NOx and reducing agent over a given cross section of the gas duct, and thus also over a given cross section of the SCR reactor, it is known to use mixing plates in the duct to cause a turbulent flow of flue gas.
However, in many systems, the concentration of NOx and reducing agent is not evenly distributed in the flue gas over a given cross section of the SCR reactor. This poses a problem since a stoichiometric ratio between the NOx and the reducing agent is essential for achieving a good reduction of the NOx content of the flue gas and a low slip of the reducing agent from the SCR reactor.
DE 3723618 C1 discloses a device for mixing together two gaseous fluids in a gas duct. One of the fluids is supplied by a number of nozzles arranged in a row along a mixing plate. The nozzles are arranged at an angle with regard to the mixing plate and the main direction of fluid flow through the duct, whereby the supplied gas is injected into the turbulent flow downstream of the mixing plate.